Ads 468x60px

| share


Plate tectonics continental drift theory

Plate tectonics and continental drift theory

        Plate tectonics was born in 1968 in the form of a theory of the evolution of the Earth's surface as a whole, hence the name given to it often global tectonics. Based on geophysical approach, it has since been the subject of numerous checks that have made it a widely accepted theory, making obsolete all other theories orogenic, or at least reducing the explanation of partial processes (for example, the gravitational slip of cover tectonics).

      Plate tectonics resumes continental drift , as it was designed by Alfred Wegener (1912) and as it was abandoned for nearly fifty years under the blows of momentary geophysical arguments, despite the favor she continued enjoy in some tectoniciens, including Emile Argand (1922) was the leader and Alexander L. Du Toit (1937) one of the representatives. But it goes much further than Wegener's ideas - it is not a mere repetition - by providing them a framework and specific geophysical mechanisms persuasive impact is generally applicable to all Earth Sciences .

1. Continental drift:

      The first idea that came to mind is that geologists stability of continents and oceans, following the conventional wisdom that gives firmness to the rock eternal.

       He belonged to Wegener, a book remained famous for the ideas he has planted and the controversy it has raised (Die Entstehung und der Kontinente Ozeane, 1915), to draw attention to the possibility of moving continents, he gave many arguments, especially orders morphological, stratigraphic, tectonic and paleontological.

- Arguments morphology. Some continents fit easily into one another, like South America and Africa, in addition, this fitting is done, there is continuity of land and structures across the continental limits current. This allowed Wegener continents gather at the end of Primary, a single Pangea he opposed a Panthalassa.

- Arguments stratigraphic. Parts of continents today disjoint sets demonstrate very similar: thus will he for South America, Africa, Madagascar, India and Australia, whose Permo-Triassic series are surprisingly similar, where the idea was born of a Gondwanaland fragmented thereafter. In the same vein, the gathering of Carboniferous glacial deposits on the one hand, and coal deposits of the same period, on the other, is an argument in favor of Pangaea in the late time primary.
- Arguments tectonics. The arguments concerning the obvious tectonic shortening corresponding to the formation of mountain ranges, without that there is discontinuity between the buildings. Rather than a general contraction of the crust, you can use a combination of two fragments of it, so was born the concept of the genesis of the Alps by a rapprochement between the European continent and North Africa in South.

- Arguments paleontology. Paleontological arguments reside in the observation of similarities in faunas continents now separated: thus, in Gondwana, meet at the Permo-Triassic, the same flora and Gangamopteris Glossopteris and the same faunas Reptiles theromorphs. One can of course imagine "intercontinental bridges" now submerged. Such bridges still exist today, so the Isthmus of Suez, Pliocene connected Africa and Eurasia and has, since that time, numerous fauna passages (eg, camels, Asian, invaded Africa), so the Isthmus of Panama, which was completed in the Pliocene, has also many passages of faunas between North America and South America (even today , armadillo, native to South America, invaded North America). But must still prove the existence of such intercontinental bridges the times when they were needed, and should, in addition, comply with what we now know of the nature of the seabed, many bridges are supposed and sentenced faunas have not made their migration to the oceanic crust ... The hypothesis of fragmentation of a formerly single landmass and the relative displacement of different parts was more convincing.

       Wegener thought that the continents sialic (of sial, silicon and aluminum, the major constituents of the surface of the continents) as rafts drifting on the underlying sima (silicon and magnesium, major constituents of the ocean floor), due the Earth's rotation. The shape of the Earth is a sphere, the continents were thus derive both westward (to delay the rotation) to the equator (axifuge effect). This explained simply double orientation orogenic belts, latitudinal (or Tethyan belt mésogéenne) or longitudinal (waist circum). At the front of the continents grew by mountain ranges "bow effect" (American Cordillera) and the back had instead of "releases" of island arcs by "Stern effect" (Asian island arcs) realized that a simple asymmetry of the Pacific; reconciliation and collision of continents from the Angara and Gondwana (Eurasia and all African-Arab-Indian) gave birth to the alpine ranges of Eurasia.

        Wegener's theory was thus a harmonious and coherent. There were some imperfections in detail: for example, the Americas had a bow but not stern, Eurasia had a stern but not bow; while Africa had neither the one nor the other . But it did not affect much the overall pattern, which appeared as the first version of the geology of the overall surface of the Earth and, as such, was a great success.

       This theory, however, was rejected, probably because it allowed excess: too many continents were moved to the needs of a beetle or cave fauna such ... It was especially set aside for lack of geophysical foundations, at least in the context of knowledge of the time. Thus, Wegener would measure the relative displacement current continents (between Greenland and Scandinavia), but unfortunately the scale of displacement he wanted to highlight - very overrated though - was equal to or less than the magnitude of simple errors calculation, given the methods he used. In addition, geophysicists showed that the continents could move through simple inertia forces associated with the rotation of the Earth.

         Wegener's theory therefore fell under the blows of geophysics. Yet it is the latter which would revive: paleomagnetism should prove continental drift [cf. GEOMAGNETISM]. Thus we have been able to reconstruct the positions of the continents over time and show that they formed a unique well at the end of the primary, and virtually overlapping pattern Wegener.

       In the 1950s, continental drift had regained his credit. But it was still unable to describe how to find and the engine. The seafloor spreading and plate tectonics would answer these two questions in turn.

2. The seafloor spreading:

       The hypothesis of seafloor spreading was issued by Harry H. Hess (1960-1962) from the one hand, the shape of the mid-ocean ridges with their extensive rift median aspect, on the other hand, the existence of plans seismic discovered before World War II by the Japanese geophysicist Kiyoo Wadati, then rediscovered later by the American seismologist Hugo Benioff, whose name they bear (cf. sUBDUCTION, fig. 1). These plans Benioff dive to the outside of the Pacific bordiers continents or island arcs. It was as if, as a result of double convection across the Pacific upper mantle, ascending at the median rift, descending at plans Benioff, gave birth to the oceanic crust in the first case, and "digest" in the second.

      Such an assumption could not rely on the certainty of a difference in kind between the continental crust and the oceanic crust, which was demonstrated by geophysics. The recording of the first wave of earthquakes relatives, who walk in depth, showed different propagation speeds in the one and the other: respectively 5.6 and 6.5 km / s for longitudinal waves, 3, 3 and 3.7 km / s for transverse waves, speeds were compared with experimental measurements in granites and basalts. Hence, in a first time, the concept of continental crust "granite" and oceanic crust "basaltic". Distinction did confirm that the records of distant earthquakes longwave whose spokes pilgrim way in surface: these waves are "ahead" after crossing an ocean compared to the equivalent of crossing a continent This confirms that the oceanic crust is "faster" than the continental crust.

          "Granitic" and "basaltic" are obviously references averages. For the continental crust that covers the whole formed by the sedimentary cover, metamorphic rocks (gneiss, mica) and granites themselves. Outcrops and boreholes show, including deep drilling undertaken in the Kola Peninsula (Russia), which reached more than 12,200 meters in the early 1990s. For the oceanic crust that covers a stratified (top to bottom) of basalts, dolerites in veins or dykes complex, gabbros, cumulate peridotites, in a structure similar to that of the ophiolite massifs of the Alpine chains, considered fragments crust and upper mantle ocean swept along on the continents. Dredging in the oceans have raised these types of rock drilling have met, including that of the well 504 B program IPOD (International Program for Ocean Drilling) which, off Costa Rica, in 4000 meters of water and 400 meters of sediment, crossed almost 1,200 meters of ocean crust basalts with dolerite and gabbros (the well remains open for deeper future). Campaigns submersible dives - with American Alvin, the Nautilus Cyana and French - have recognized basalts (campaigns FAMOUS - French-American Mid-Oceanic Undersea Survey - on the Mid-Atlantic Ridge, Cyamex - Cyana of Mexico and - the ride is peaceful) or deeper words, gabbros and peridotites (Cyagor campaigns on the bench Goringe, south-west of Portugal, and Vemanaut Kanaut along the Vema transform faults in the South Atlantic and Kane, in the central Atlantic). The dual nature of continental and oceanic crust is therefore no longer in doubt.

     The discovery and interpretation of magnetic anomalies at mid-ocean ridges would lead to justify the notion of expansion at the median rifts. This discovery was made by Ronald G. Mason (1958) in the Pacific west coast of North America, it was quickly confirmed and extended to other oceans, including the Atlantic Ocean at the Reykjanes Ridge, south of the Iceland, which is the northern part of the Mid-Atlantic Ridge.

      These anomalies were interpreted by Frederick J. Vine and Drummond H. Matthews (1963), taking into account palaeomagnetical thermorémanent: bands magnetic anomaly, rift in parallel and symmetrical with respect thereto were interpreted as being due to many successive intrusions from basic material of the mantle, while each the paleomagnetic field was the same direction as the current field (positive anomalies: the thermoremanent magnetic component is added to the current field), while the other paleomagnetic field was opposite to that of the current (negative anomalies: the paleomagnetic thermoremanent component is subtracted from the current field).

       This being admitted, we can then, depending on the calendar used paleomagnetic, assign an age to each band of magnetic anomalies, thus showing that the deep ocean is more ancient, as one moves away from Median rift. There arise where these cards, along with rifts, identifies key anomalies identified by their serial number in a fixed schedule or even their absolute age from the calendar.

      At the same time, the calculation of the rate of expansion - indeed half-rate because the phenomenon is valid on both sides of the ride - is made possible by simply dividing the distance a band rift anomalies by age it: values ​​obtained range up to a maximum of 17 to 18 centimeters per year at the East Pacific Ridge in the north-equatorial. The most frequent value, especially in the Atlantic, is about 2 centimeters per year.

     The analysis of earthquakes occurring at mid-ocean ridges clarified the terms of the hypothesis. This is John Tuzo Wilson (1965) noted that the seismic characteristics of the major accidents to cross mid-ocean ridges. If they were real setbacks, we should expect that households have seismic over the entire length of these accidents. But it is not so: the seismic foci have only the section of the accident between the two offset portions of the rift. If, instead, we consider that these accidents, shift away from a rift preformed are created by the same process as that one, we get a very different result: the rate of seafloor spreading is assumed the same on both sides of the accident, we immediately see that, between the two offset rift, the direction of relative movement of the oceanic crust is opposite to that of the apparent setback, while beyond the rifts, the game equal rate of expansion oceanic crust movement is in the same direction and the same speed of both sides of the accident, which thus can be considered dead. We understand that the only seismic foci are located between the rifts. These accidents were designed and called transform faults by J. T. Wilson, to distinguish them from setbacks.

      Lynn R. Sykes (1967) was to undertake the verification of this fact by a new analysis of earthquake foci mechanism to mid-ocean. If we accept that an earthquake at the hypocenter is caused by a sudden rupture along a fault, the stress state corresponding to this failure can be divided into four quadrants, two compression and two expansion, these two situations by expressing the meaning of the first movement and the overall result of it. If so, on the occasion of a given earthquake, we took stock of the meaning of the first movements in different seismological stations distributed throughout the world (using P waves arriving early), they must be divided into four quadrants separated by two large circles perpendicular to each other, such that, in pairs, one can observe one.

      However, this result does not mean, reversing the reasoning to determine the cause of the fault and earthquake motion. If you do not know the fault that causes the earthquake, there is an uncertainty of 900 in the possible direction of this: we can not choose between the two families combined with a network of faults and, by consequently, we can not determine the motion. But if you know the cause of the earthquake fault - so if you know which case 7 b and 7 c is good - we can determine the motion along it. L. R. Sykes applied to earthquakes so this method of mid-ocean ridges, considering that they occurred along transform faults, and thus confirmed that they correspond to a reverse movement of the apparent setback, which was the assumption of J . T. Wilson.

No comments:

Post a Comment